The present invention relates to machine printing of color positives from photographic color negatives, and more particularly to improvements in apparatus for continuously influencing the color of light beams in photographic color printers. Still more particularly, the invention relates to improvements in apparatus for selectively moving portions of several color filters, into the light beam which impinges upon photosensitive material in a printer.
Color casts in photographic printers are normally removed (a) by selecting the length of intervals during which corresponding filters extend into the light beam or (b) by changing the intensity of the filtering action. The first technique necessitates substantial expenditures for equipment which is used to move filters into the light beam while the latter impinges upon photosensitive material; moreover, such technique presents serious problems when the exposures are being made in rapid succession because the interval during which the filters are being moved into the light beam cannot exceed a predetermined fraction of the exposure time.
The second technique (intensity regulation) necessitates the utilization of packs or stacks of filters which are stored in a so-called filter drawer and whose transparency or light transmissivity is proportional with the degree of cast which is to be removed from the light beam. Proper filtering of printing light necessitates the utilization of groups consisting of large numbers of filters in each of three complementary colors, and the density of filters in each group varies only slightly from filter to filter so as to insure that each group will furnish a complete spectrum of different densities. The initial cost of such groups of filters is high and the removal of color casts by resorting to groups of filters takes up a substantial amount of time. Therefore, the just-described intensity regulation technique is employed practically exclusively in relatively or extremely simple printers wherein the output is of secondary importance.
It is also possible to employ filters of maximum density and to install such filters in an unoccupied (structure-free) plane of the beam of printing light. Depending on the desired filtering effect, selected filters are moved into such plane to a greater or lesser extent. An unoccupied plane which is highly satisfactory for the mounting of movable maximum density filters therein is the focal plane of the objective because a filter which is mounted in the focal plane can influence all points of the copying plane to the same degree. However, many printers do not provide enough room for the mounting of a complete filter assembly in the focal plane of the objective. It is therefore customary to place the filter assembly immediately behind the light source in front of the positive. In order to homogenize the light beam before it reaches the positive, a mixing duct is placed between the filter assembly and the copying plane. The purpose of the duct is to insure a uniform distribution of light in the three colors across the entire beam of printing light. The length of the mixing duct is greater if the distribution of colors in the light beam immediately behind the filter assembly is non-uniform, i.e., the duct can be made shorter if the filters influence the beam in such a way that the beam is substantially homogeneous as soon as it has passed through the selected filter or filters.